1. Technical Field
Several aspects of the present invention relate to a semiconductor device, and in particular to a structure technology for improving the electrical characteristic of a thin film transistor.
2. Related Art
The semiconductor device described above is provided with, for example, a thin film transistor (TFT) formed on a glass substrate. In detail, a semiconductor layer is formed on the glass substrate, and a source region and a drain region are provided to the semiconductor layer. Further, the semiconductor layer is provided with a gate electrode formed thereon via a gate insulation film. It is known that in such a semiconductor device, in order for improving reliability of the electrical characteristic, there is provided a lightly doped drain (LDD) structure having a low concentration impurity layer in the source region and the drain region of the semiconductor device or a multi-gate structure composed by connecting a number of gate electrodes. This structure is described in JP-A-2000-216397, for example.
However, in the semiconductor device having the LDD structure, since the load (resistance) becomes large because of the low concentration impurity layer, the output current is reduced because of the voltage drop when a voltage is applied to the thin film transistor. On the other hand, in the semiconductor device having the multi-gate structure, the electric field tends to be concentrated (increased in intensity) at the end portion of the drain region, and the increased electric field intensity might cause an abnormal leakage current in the drain region or dielectric breakdown.